1. Field of the Invention
The present invention relates to a mobile communication method, a mobile communication system and a radio network controller with which a radio base station notifies a mobile station of transmission power control information via a transmission power control channel, the transmission power control information used in the mobile station to control transmission power of uplink user data.
2. Description of the Related Art
As shown in FIG. 1, in a mobile communication system defined in 3GPP Release 6, each of radio base stations NodeB #1 and #2 is configured to notify each of mobile stations UE #1 and UE #2 of transmission power control information via a transmission power control channel (F-DPCH: Fractional Dedicated Physical Channel), the transmission power control information used in each of mobile stations to control transmission power of uplink user data (see 3GPPTS25.211 V7.2.0).
Here, a method for notifying transmission power control information via an F-DPCH will be described with reference to FIGS. 2 to 4.
As shown in FIG. 2, each radio frame (10 ms) in the F-DPCH is formed of 15 slots (slots #0 to #14).
The structure of each of the slots in such a radio frame is specified by a table shown in FIG. 3.
In FIG. 3, a “slot format” shows an index for identifying the type of a format of each of the slots. In 3GPP Release 6, only one type of F-DPCH slot format is defined. However, 3GPP Release 7 supports ten types of F-DPCH slot formats specified by the difference of the offset of the F-DPCH symbol (transmission power control information) from the head of each of the slots.
A “spread code” shows how many chips form one symbol in each of the slots. As shown in FIG. 3, one symbol is formed of 256 chips in a F-DPCH slot.
A “bit pattern” shows a bit pattern transmitted in each of the slots. As shown in FIG. 3, either of “11” or “00” is transmitted in an F-DPCH slot.
A “channel bit rate” shows a bit rate in the F-DPCH. As shown in FIG. 2, only one F-DPCH symbol is transmitted in a F-DPCH slot, and 15 slots (0.66 ms) are transmitted in a radio frame of 10 ms. Accordingly, as shown in FIG. 3, the bit rate in the F-DPCH is “3 kbps (=15 slots×2 bits/10 ms).”
A “channel symbol rate” shows a symbol rate in the F-DPCH. The F-DPCH is modulated by a QPSK system. Therefore, the symbol rate in the F-DPCH is “1.5 kbps,” which is a half of the bit rate in the F-DPCH.
Here, each of mobile stations UE is individually assigned an F-DPCH. In addition, each F-DPCH is specified by a channelization code unique in each sector.
Furthermore, as shown in FIG. 4, each F-DPCH specified by a single channelization code is shared by ten mobile stations UE #1 to #10 with time-division multiplexing.
In FIG. 4, F-DPCH symbols #1 to #10 for the mobile stations UE #1 to #10 are configured to be transmitted respectively at transmission timings T1 to T10 defined in accordance with the offsets from a PCCPCH (Primary Common Control Physical Channel).
Furthermore, with reference to FIG. 5, a description will be given of a method for assigning a channelization code (CC) and a transmission timing for transmitting a F-DPCH symbol to a mobile station UE in a mobile communication system defined in 3GPP.
For example, consider a case where a mobile station UE starts communication via a radio base station NodeB #1 in a sector #n. To handle this case, as shown in FIG. 5, a radio network controller RNC is configured to randomly select a transmission timing #6 from the transmission timings usable in a smallest channelization code CC#1 having a smallest identification number among usable channelization codes in the specific sector, and to assign the smallest channelization code CC#1 and the transmission timing #6 to the mobile station UE.
In addition, consider another case where the mobile station UE adds a radio link with the radio base station NodeB #1 in the sector #n or NodeB #2 in another sector #n+1. To handle this case, the radio network controller RNC is configured to assign the smallest channelization code CC #1 and the transmission timing #6 for a F-DPCH when the aforementioned communication starts.
However, the aforementioned mobile communication system has a problem when the mobile station UE #2 performs handover from the sector #B1 of the radio base station NodeB #2 to the sector #A2 of the radio base station NodeB #1, as shown in FIG. 1. More specifically, in this case, if a transmission timing TA and a smallest channelization code CC #1 that are assigned to the mobile station UE #2 before the handover are also assigned to the mobile station UE #1 in the sector “A2 of the radio base station NodeB #1, a F-DPCH symbol transmitted to the mobile station UE #1 collides with a F-DPCH symbol transmitted to the mobile station UE #2.